For most of my adult life, I've kept track of the date that the maple trees around me first provide shade as their leaves open in the spring. I've noticed that it is happening 2 or 3 days earlier now than it did 30 years ago.
The University Corporation for Atmospheric Research (UCAR) is sponsoring a truly interactive project to monitor when the leaves and flowers of certain plants "pop" in the spring, and they're looking for help from, well, everyone! The project is called BudBurst, and the 2008 season kicked off on February 15. Everything you need to know is on their web page, but essentially you'll be watching specific species of plant, and reporting the dates of bud burst, flowering, etc on an interactive web page. I've already registered, and picked several plants in and around my yard to monitor. The Fox Lane Campus is a great place to find many of the species listed at the BudBurst site, too.
The picture accompanying this post is of pollen released from pitch pines on Schunnemunk Mt. near Newburgh, NY. It was windy the day I was up there, and I came home with a yellowy green tint on my clothing and hair from all the pollen blowing around up there. Pollen release is one of the events the BudBurst team is looking for.