Right now, there's an incredible diversity in company websites. Some include significant information about the history of the group, their past and current repertory, their artistic staff, their budget and financial information. Some barely include this season's performance schedule. My hometown company (Pacific Northwest Ballet) does pretty well -- they've got substantial information about the current season, links to some good video, online versions of the bios we see in the programs, and even a couple years' worth of annual reports on their website. But although they have a mostly-complete list of "current repertory," it doesn't always include information about when the company performed the work, and it doesn't list works they no longer dance, even if they were mainstays of the company in earlier days. And they don't always include program essays or other work -- their archivist wrote a great series of short essays this year for their anniversary, and not only are they not on the website, they weren't even in the regular program -- they were passively distributed in the lobby. If you knew they were there, you could pick them up, but they didn't think to promote them in the program that I saw.
The Twyla Tharp website has been recently buffed up, and it's excellent. There are entries for each of her works, including premier dates and casts, subsequent performances, excerpts from press coverage, still photos, and in many cases, video clips. If only more artists and companies were able to do this kind of work.