There are so many cool Web apps out there, it is hard to just talk about a few. I encourage you to try any new Web app that you think might help support teaching and learning and add your favorite(s) to the comments below!
Alltop.com presents a categorized page of RSS feeds in all types of categories, education included. It is an easy way to quickly get feeds from the most prolific, informational, and news-worthy blogs. When trying to explain the concepts of RSS and aggregation during a professional development session for teachers, I found demonstrating Alltop a good way to introduce them to those topics by letting them investigate the site.
Voicethread has really become popular with educators. A Voicethread "is an online media album that can hold essentially any type of media (images, documents and videos) and allows people to make comments in 5 different ways - using voice (with a microphone or telephone), text, audio file, or video (with a webcam) - and share them with anyone they wish. A VoiceThread allows group conversations to be collected and shared in one place, from anywhere in the world." It is easy to use, powerful, and a lot of fun!
Twitter is a wonderful tool. Imagine having access to all your personal and professional friends and colleagues for help and support all of the time! Educators are using Twitter to share and gather facts and information from other educators all over the world. Users keep a client open on their desktop, and another user can post a question (I asked my education friends about their school-sponsored student e-mail naming conventions yesterday before deciding what ours would be) and the wonderful, helpful education community always comes through with plenty of suggestions and thoughts! Users can write to you directly or answer the question by directing the answer to you but allowing others to see the answer, too. The "tweets" are limited to 140 characters, so I have become a very succinct question and answer creator!
These three are enough to keep you occupied for hours-- have fun and please share your new discoveries with the rest of us in the comments!