Buying a new computer teaches you a lot about essential software: if you REALLY need it, and you've REALLY enjoyed using it in the past, you'll shell out some money to buy ANOTHER copy.
Two pieces of software that I can't live without -- and that I've been happy to buy a second time -- are Audio Hijack Pro and iDrum.
Audio Hijack Pro does exactly what it says: it hijacks audio from your Mac applications. So when I'm listening to an old time radio program with Quicktime and I decide I want to record one of the program's songs, I simply hijack the Quicktime output and save an MP3 file. If I want to quickly record input through my iMac's line in source, I hijack that instead and save an AIFF.
I have never met an application that Audio Hijack Pro couldn't handle. It has never crashed on me or corrupted a file. It even allows me to insert Audio Unit effects into the recording process if I like, which allows me to clean up cassette input while recording it. Very cool and convenient.
iDrum is a nifty drum machine that can operate as either a plug-in or a stand-alone application. Its drum samples are simply audio files, so if you don't like their drum sounds you can add your own. It's totally stable, computationally inexpensive, and VERY friendly to use.
It may not be "Ultrabeat," but you don't always NEED Ultrabeat. Sometimes you want to make a quick drum pattern, and that's what iDrum does perfectly.