I recently got one of these, except mine is a garden sprayer from Lowe's:



If I bathed my bike on a regular basis, I'd probably go with something bigger that provided a bit more pressure for the fine spray (like Sarabeth's). But for a once or twice a year, serious frame cleaning, it worked nicely. I took the bike out on my patio, sprayed it down, used a bucket of soapy water and sponge to wipe off last year's dried mud, then used the sprayer again to rinse off the soap. I used the finest spray near the bottom bracket, and then adjusted it to get a harder stream into spots like the inside of the fork.

Very handy if you're short on space like I am, and a deal at $10. I can even imagine throwing it in the trunk of the car and using it after a muddy ride. It'd also work in the bathtub scenario that fidlfreek described if you don't have a handheld shower head.