There aren't many Singles 2 sites out there in English (several in German and French), but this page is kind of obligatory, so here goes (all links open in a new window or tab, so make sure your pop-up blocker is turned off):
Deep Silver: the folks who brought you the game (the link is to the English homepage)
Rotobee: the website of the game's developers (it's in German)
Singles Forum at Deep Silver: Tips, Hints, Help, News and whatnot; be nice and they'll be nice to you. Puddy is the current moderator. Be nice or all your base will belong to him.
Singles 2: the official website; but the forums moved to Deep Silver's site
CafeMax's Singles Blog: She was the moderator on the English forum at Deep Silver, but has since moved on to other pursuits (*sniff* - miss you) - articles are in both French and English (just click the American flags if you're seeing French) - she's a very nice lady, so behave.
Singles 1: the official website for "Singles: Flirt Up Your Life"
FAQ at GameFAQs.com: sbermeister's Singles2 FAQ - it's incomplete, but covers Josh's story pretty well.
There are a couple of groups for Singles 2 at groups.yahoo.com, but except for a flurry of activity when the game was released, they've pretty much died off. Perhaps they'll come back up, perhaps not.
A note on extensions in general: All of the extensions that you'll find through these links come in .zip files. If you are running Windows XP, Vista or 7, it will handle them natively and there is nothing else you need to extract them. If you are running any previous version of Windows, you will need an unzipping utility. There are over 100 different unzipping utilites out there and most of them are free. Check out Shareware.com or Download.com, but make sure that whatever you get can handle .zip files.
Once you have downloaded the extension and uncompressed it, place the uncompressed folder into your \singles2\extensions folder and you should be ready to go (the Booster has a texture file replacement, but the rest don't). For furnishings, it has been my experience that these items never show up on the main menus in build-mode, but do show up when you look at particular categories. So just hit the appropriate category button(s) and you should find your new furniture. Clothing seems to show up without a problem. None of these extensions are native English, so perhaps it's just an oddity that involves the game's language files. Note that some of the extensions install new features and menus into the game and most of the ones from the German sites are in German. So if you don't read the language, you'll either have to do without or play with it until you figure out what it does.
Singles 2: Triple Trouble v.1.4 Patch: If you bought the digital download version of the game from direct2drive, your game should already be patched to v.1.4. If you have a retail CD version, you probably need this. If you have a pirated or cracked version, I hope it fries your system, but it will probably work OK. The patch was not tested against every language-version of the game, but should work on all of them. You might need to reinstall or update your DirectX9 files. There was a DirectX9 update released between the 1.3 and 1.4 patches and it checks for what it thinks is the current version (we're up to DirectX11, but it doesn't know that).
The patch fixes some major problems created by the 1.3 patch (like crash-and-burn on loading a saved game). It supposedly disables a few of the copy-protection features, but is otherwise what 1.3 should have been. Considering that it has been about four years since its release, I doubt that there will be any more updates to the game.
Singles 2 Booster: Adds some new sexual activities, removes a lot of the nudity restrictions (in photo mode, for example), lets you control NPCs, and whatnot.
Steffi1964 wrote the Booster, but has since taken his site down after releasing his code for redistribution. You can download a 7zip version of the Booster here. The page is mostly in German, but it's fairly easy to figure out what's what. For those without the capability of handling .7z files, I put everything into a standard zip, which can be downloaded here. I am hosting the .zip file, but that's my entire contribution to the effort.
The 2.5 version of the Booster incorporates work by several players in addition to Steffi1964's work. Appropriate attributions are in a text file in the main folder. I am not a programmer, but if I am correctly understanding what I'm seeing, the "Booster Optionen" may already be incorporated into v.2.5.
The "Singles2 Booster Optionen" utility lets you turn cheats and features on and off without having the game running, but will not work without the Booster being installed. Just unzip it into the extensions folder and then run the executable - nothing else to install, but you really do need to be able to read German to use it. The .zip file is hosted here. CAUTION!!! The Booster Optionen is older than the v.2.5 of the Booster. Install it at your own risk.
Phalzyr has created an add-on to Steffi1964's add-on. You can read about it here. The appropriate links are in the forum thread. I have not used it, so can't say much other than "read the post". A word of caution, though. Phalszyr's post says that this is for v.2.4 of the Booster, so no guarantees as to whether it will work with the current version. The 2.4 version can be downloaded from the same page as 2.5, at least as of May 2011.
Bonsai of Life: this one is hosted in a couple of places, but I don't mind burning Deep Silver's bandwidth. Unzip it into your extensions folder, then purchase the Bonsai of Life (in Domestics and Appliances: Candles and Decorations) and place it somewhere that will be easy to click on. Add some quick money, experience points, skill points, manipulate needs and relationships, add other Singles as roommates, etc. I've found this one to be the most fun to use, especially in the penthouse where you have lots of room to expand. Just in case Deep Silver takes the file down, you can get it here.
Character Changer: This is a great little utility by SteveXY, who hosts the singles forums at Singles-foren.de. The forums themselves are in German, but it's pretty easy to figure out what to click to download it. You will have to register a username to download it from the forum page (may take a few minutes for the confirmation email to arrive), but he won't spam you. Anyway, he's added a good dozen or so new female meshes (no males, sorry) and has done a mighty fine job with it. The utility runs outside the game and has German, English and French menus (click the appropriate flag to choose the language). It works great, but whatever you do, don't delete the character changer until you have restored all of the original characters or you'll lose them (it completely replaces the character meshes in the game folders). There is a third expansion pack that you can add. Unzip it to the \charchanger2 directory. The Character Changer is about 23MB (zipped) and the expansion pack is a bit over 5MB (zippled). Unzipped, they are a bit more than 57MB.
I have both files but will not make them available unless (1) the singles.foren.de site goes away AND/OR (2) SteveXY gives me permission to make them available.
Many of SteveXY's extensions are also hosted at CafeMax's site. Since her site is also in English, you might find it a little easier to read the descriptions. Look for the "Triple Trouble" link under "Additions" on the left side of the page.
One particular utility at CafeMax's site is the "Book of Love" (or perhaps "Book of Relationships" - my French is fairly horrid). It does some of what the Bonsai does, but only affects relationship levels. I'm linking it here, but I think you'll find the Bonsai more useful. Just extract the .sx2 file to your \extensions folder.