As Shrui said, it's all about what you want to get from it, though I figure you could do with a few more specific pointers.
He's already covered the basics, iLife for media and its general manipulation, Windows for gaming, and a new mouse. With the new mouse, I recommend getting one, even if you've got the latest one. The wired one with the ball sucks badly, and the new one is a bit snazzier, but inherited the same fatal flaw. You will always have to raise your index finger in order to right click. It's the single most aggravating thing I've encountered with Apple's products, and its the reason I abandoned their mice a looooong time ago.
Anyway, back to the apps. The go to places to get them are macupdate.com or versiontracker.com. You can pretty much just type in what you need from the app, and it'll fine 30 of them for you to try. So, here's a list of things to check out.
RSS reader – Netnewswire. I don't like the built in RSS stuff Apple shoved into safari or mail, so this is what I use.
emails – mail. Now that hotmail puts out POP3, you can use this with pretty much everything.
web browser – Safari, but Firefox if you want free plug-ins. All the plug-ins for Safari (Saft, Glims, ClicktoFlash are the best 3) are basically hacks.
Download manager/FTP – I use Speed Download and Transmit for these tasks. SD is great for letting you queue up downloads, restart them, and whatever, whilst Transmit is the best FTP file manager out there.
Password manager – 1Password is probably the best piece of software I own. It can keep a hold of every single one of your passwords (which it helps you cook up so they're a random stream of gibberish), all your software licenses, card details, identities, and lock it all up securely for you till you need it at a simple keystroke. Simply it's just awesomely useful and just hides away in the background.
backups - I recommend either CarbonCopyCloner or SuperDuper, both of which will give you a pure clone of your disk, and (SD is paid for) allow you to do incremental updates to those clones so that it doesn't take forever. I dislike Time Machine for anything but versioning. It doesn't help that over time it slows down to a glacial pace trying to keep track of everything, and will eventually destroy that HDD's partition. Even if you like TM, I'd still say to clone your disk aswell, since it's bootable and can get you right back into things very very quickly.
Firewall-esque – LittleSnitch. I really love this little app. It follows rules you set yourself (on the fly) about allowing apps to connect to the internet. It can be annoying initially, because everything loves the net these days, but I love it for letting me keep tabs on things and not waste my bandwidth. Use it in tandem with your built in firewall.
Digital library – I don't know if you obsess about things like me, but I adore Delicious Library. The first thing I do when I get a BD or a Game is to scan it into this and have it as part of my little catalogue. Probably quite pointless, but delicious nonetheless.
iPlayer – search for a little app called iPlayerDownloader. It's probably a bit naughty of me to say this, so I'll throw in a helpful [REDACTED] here for the mods so they don't have to write it themselves. It does what it says on the tin.
All video files ever – VLC. It's cross platform, so you might have heard of it, but it'll play anything you can throw at it, and do it better than any other piece of software.
*phew* sorry if that's a bit of a wall of words (sorry, I dunno how to embed links into words), but it's a lot of the everyday background software I use for some fairly menial tasks. A lot of them are paid for, and I can't really remember how much they were, but I couldn't live without many of them.