while ago... and here it finally is!
A lot of people give up on biking (or doing much outdoors at all) when it rains, which is unfortunate since in the Washington D.C. area, it rains an average of 87 days per year (or roughly 7 days per month). If you have the right equipment, dealing with rain isn't that tough at all.
In terms of bike commuting, the first thing to remember is this: it doesn't matter nearly as much if it rains on your way home. There are clean, dry clothes at home (or at least, there should be... if there aren't, stop reading this and go do some laundry!). It's rain on the way to work that gets tricky.
Here's my shortlist of the essentials for biking to work in the rain (and enjoying it):
1. A change of clothes. A towel might be good too. You can keep as much of this at work as you want... I generally have a towel and few extra clothes at work, but mostly take my day's attire with me each day.
2. A waterproof bag for your clothes. This can be a messenger bag, pannier, or - in my case - trunk bag with a waterproof shell to go over it. I keep the shell (which is basically a raincoat for the bag) in one pocket of the bag, so it's always there when needed (you can see this in action in my "Snow Bike" post).
3. Rain jacket/pants. These don't have to be bike specific. I use a Marmot Precip jacket that I got for backpacking, and a basic pair of wind pants - not technically waterproof, but definitely highly water resistant.
4. Wool socks. My shoes aren't waterproof, but it doesn't matter as long as my feet stay warm - which they will with wool socks. There are also some tricks or equipment you can use to dry your shoes after they get wet.
5. Fenders. I have Portland Design Works Soda Pop fenders - a great deal at $25 for both, and also nice because they're easily removable for when I know it's not going to rain. Fenders not only help keep me dry, but also help keep a lot of road gunk off my bike and off me.
That's really it. If you have those things, you can add whatever other clothes are seasonally appropriate under them, wear gloves if it's cold, etc. Even if you don't have all these things, it's not a deal-breaker - I didn't have fenders or wind pants until recently. However, these things do make it so I'm much more likely to bike in the rain.
Disclaimer: I have not been paid, bribed, threatened, or otherwise encouraged to promote specific products or brands in this post. The opinions expressed are my own and are based on my experience - nothing else!
Child on tricycle image courtesy http://wn.com/recumbent_bicycle, used under the Creative Commons license.