Booting multiple distributions
Unless you're faithful to a particular distribution, sooner or later curiosity will get the better of you and you'll likely want to try another flavour of Linux on your Pi.
To avoid the hassle of swapping SD cards, you can take advantage of XECLoader. It's an experimental bootloader for the Pi which presents a customisable menu of boot options.
Before you start slapping XECLoader into /boot and restarting your Pi, you should know that it's not for beginners. Kernels used must be stripped of the first32k, and your mileage may vary getting Raspbian up and running ( I've failed thus far ). If you plan to boot from an external hard drive, you'll also need to make sure your Kernel supports this.
At time of writing I've successfully got both Debian and Fedora up and running, although Fedora is a little flaky.
From my assorted tests, an external hard-drive gives marginally better performance than a good SD card with tweaks, but you pay for this with a loss of portability and an increased power footprint. I've been fiddling with scripts to detect the lack of a hard-drive at startup, and boot from the SD-card instead but haven't yet come up with something polished enough.