I too do exactly the same. Most of the time it's some quick rails app[1] or some server side scripts that do something I never did before (like parsing mails, pg stored procedures or some jquery[2]).
Do you have some links to your side projects/no-burnout-work?
[1]http://diffuse.it/ (a blog that is actually used by me, my girlfriend and 2 friends :))
Sure (although a lot are websites that were shut down later on, to focus my resources):
[1] http://www.learnivore.com (ruby,rails,iphone screencasts aggregator)
[2] http://www.toutpourmonipad.com (ipad case finder for the french market)
[3] http://github.com/thbar/opaz-plugdk (an experiment to create VST plugins using JRuby and Java)
[4] http://github.com/thbar/music-labs (more music stuff with ruby)
[5] http://github.com/thbar/petite-lettre (a little ruby gem for inter-process communication based on yaml)
[6] http://github.com/thbar/magic (a DSL to make it easier to implement UI with IronRuby, WPF and Windows Forms)
I have one side-project ongoing (that I will mention here later on) and another one for after.
I have learned a lot on each of these projects - bits that have most of the time been reused shortly after on customer projects (sphinx, mongodb, jruby...).
When working on a long/difficult project A, I keep one smaller but equally interesting (ROI-wise) project B on the side, and ensure I procrastinate in a time-boxed fashion on the useful project B only (vs. more classical procrastination).
The time-boxing ensure this stays at the hobby level, and regularly project B becomes difficult too to kick you back into project A.
It also gives me some times to relax and think about the issues on the other project.
To be used with careful time tracking of course :)
This removes a bit of the burn-out feeling, while still working on something that moves us along.