I am definitely a product of a generation that uses technology and the internet to the fullest so its no surprise that I use a lot of different online sources to get information that I need for various projects I have going on both for work and my own independent learning. Whether you are a biologist, a botanist, a forester, or a parks manager there is a litany of great online sources that can be your best friend.
One that I find to be one of the best and which was recommended to me by a professor at the University of Montana is eNature.com. This particular website is like having every single publishable field guide in the world at your fingertips. It has a little bit of everything for everyone. I use eNature.com primarily for recording my observations from day to day and doing preliminary research on species habitat and behaviors before going out in the field. If you are interested in birding this site also presents sound bites of every bird species' call and discusses everything from diet to wing patterns when flying. Again, when I was taking ornithology this site came in handy but I've continued to use it ever sense. Signing up for an account is quick, easy and free. Over the years I've built lists of animals of interest for the habitat and region in which I work. I've also recommended this site to a number of silviculturists and foresters in my office because of the section dedicated to plants and plant disease. If you have kids that are interested in the outdoors I suggest bringing them to eNature.com as soon as they can be trusted with a computer because there are tons of educational activities and games that help them to learn about everything from rock formations to determining what scat came from what animal. All in all, you really can't go wrong with this one just do a little exploring on the site to see all the resources it contains.
Another site that covers some similar bases as eNature.com, called WILDpro, is actually under authorship of the USGS. This is a free membership site that puts out information on hot topic issues such as Westnile virus, chronic wasting disease, and wildlife first aide and care.
For those of you with more academic interests, say publishing papers or at least going through the peer review process, the best tool you are going to ever have in your back pocket is some sort of scholarly article search engine. Now most of the better engines require a small fee for their services but they are well worth it. I'm a fan of Proquest and DOAJ (Directory of Open Access Journals) but its just because the are the ones that I have been using for a long time and both cover a variety of journals. Another commonly used engine is JSTOR, but again the membership can get pricey. Most education and work organizations have contracts with specific academic search engines but if you find that your school or place of work does not and you don't want to pay an annual membership fee both Google and Yahoo have developed "scholar" settings on their search for just this situation. You may not always get full text articles but often enough if you have the citation you can go to that specific journal's webpage and look at back issues to find the full original text. This process takes a bit more leg work but its just as effective.