I've recently run into some space issues on my primary partition on my home Mac Mini. Not really problems, since I've got 200GB of additional storage attached to it, but you never want to have a primary partition wanting for space (on any system, as you need that "free space" for the page file/virtual memory).
On Windows machines, I recommend using a tool like SpaceMonger to profile your hard drive(s) and delete unwanted files, etc. (Note - NEVER DELETE a file unless you know what it is, what it does, and that it's unnecessary or redundant)
On my Mac, I had already analyzed my disk, moved or deleted things that didn't belong on my system drive, but I still didn't have as much free space as I'd like. And that's when I discovered Xslimmer.
Most (90%) of Mac programs are now "Universal Binary" programs, meaning that they can run on older, PowerPC-powered Macs as well as new, Intel-powered machines. Which means (basically) that there's two sets of code on every program that's on your hard drive. Additionally, Mac programs often ship (download) with multiple language packs to support a broader range of users. I only speak English, so I rarely (never) need to run a program in Spanish. Or Dutch. Or whatever they speak in Kazakhstan.
What Xslimmer does is analyzes your applications folder, and strips out the code you don't need. If you have a PowerPC Mac, it will strip out the Intel code from your apps, and vice versa. It also removes unnecessary language packs from your apps. The initial analysis and "slimming" took about an hour (during which time I was still able to work without any memory hit) and it saved me almost 4.5GB of space!
So if you've got some Mac bloat (and lots of applications), Xslimmer might help your Mac fit into its thin jeans again!