You've gone through all the trouble of making a mod; don't just keep it to yourself. Let others enjoy your hard work. Although your mod will probably get more exposure on a popular web site listed below, don't be afraid to upload to your own web site if you want; Google searches can often lead people to smaller sites for addons.

The most popular sites to upload L4D2 addons are:

1. L4DMaps.com

2. L4D2 Workshop on Steam

3. GameBanana.com