My phone, still a t-mobile G1, is pretty much an antique at this point - I believe it was the first android phone. I installed a corporate email app for work and ran out of space - wound up having to delete a bunch of apps and even then periodically reset the app as it filled up my phone's memory with email. The performance of the phone was dismal - I'd often find myself waiting for 10 seconds or more to see a response to a button push or screen tap.
I rooted the phone a while back, and this added a bunch of functionality, but didn't in itself solve the problem. I was aware of several scripts in distribution to make the phone store more of its internal data on a SD card (I've got a 16 gig SD card in mine), but the age of the G1 hurts here, too, as the maintainers of the scripts have moved on to newer and better phones.
Finally, I took matters into my own hands, and, at the prompting of Tim, created a couple of new partitions on the card - one linux file system type (at first formatted to ext4) and one linux swap. I started with really big partitions, but the phone wouldn't recognize the card - I wound up with a 512 meg linux partition (ultimately formatted to ext2) and a smaller swap.
Although the phone was already using the SD card for certain apps, I found that I could manually mount the ext2 partition and move additional stuff there using symlinks. I have been unable to mount the ext2 partition automatically at boot time (even though I see an init.d script that appears to be written to find and mount it automatically - not sure what's going on there), so I just wrote some scripts I run at runtime to set things up properly, and it's worky, but it's OK.
After freeing up about 15 megs of space this way, wow, the phone is really responsive and zippy again, and I'm even able to put back on some apps I had removed, like a gmail upgrade and DocumentsToGo.
Also, an app in the market showed up that lets me view my home security cameras from the phone - finally! The supports linux motion, which I'm using, which makes set-up a lot easier - otherwise, I think it would probably support all the cameras I have individually, but I'd have to punch each of those through reverse NAT, and I think security would suffer - most or all wouldn't provide SSL support.
Anyway, long live my G1!!!