Most experienced amateur astronomers will already have this information, but the hobby attracts newbies all the time who have to learn everything from scratch. So if that latter description fits you, here are a few websites you need to know about:Heavens Above - www.heavens-above.com
You have to register to make best use of the site, but it's well worth it (and free). Enter your latitude and longitude and you'll be able to look up sun and moon data for your location as well as transit times for various bright satellites, most notably the International Space Station (ISS) and Iridium satellite flares. Most people use this site to track satellites, or to identify a satellite they saw after the fact. If you haven't seen an ISS pass, it's well worth it, especially if you know exactly what time to look up and can persuade your dinner guests to step outside for a few minutes.Cloudy Nights - www.cloudynights.com
Cloudy Nights is the top telescope review site on the web, although it covers more than just scopes. Volunteers write reviews on everything from astronomy software to eyepieces to CCD cameras. The reviews are available to anyone, although you have to register to post to the classifieds and the bulletin board. Definitely check this site out before you buy.AstroMart - www.astromart.com
After you've read the reviews on Cloudy Nights and know what equipment you want, you'll definitely want to check out AstroMart, which has got to be the largest online astronomy buy-and-sell community. Registration is required and free. Most experienced telescope buyers will avoid eBay for astronomy gear because eBay tends to have nothing but department store telescopes that are being sold by people who never used them. On AstroMart you'll usually get a fair to excellent price on used equipment that has been thoroughly vetted (and often upgraded) by the owner. The site also has paid classifieds from commercial retailers selling new equipment and an active bulletin board and review section.SkyMaps - www.skymaps.com
The SkyMaps site publishes monthly sky calendars that list the best small-telescope objects for that month. While not particular useful if you have already progressed beyond the basics, SkyMaps are perfect for children's groups or sky tours and they allow free distribution to educational groups and individuals. Also, they have a really nice online store with astronomy books, atlases, and posters.Clear Sky Clock - cleardarksky.com/csk
The Clear Sky Clocks page is one of the most valuable resources to active astronomers that you've probably never heard of. It's basically an astronomy weather page that is uncannily accurate down to the hour. A totally home-grown effort by programmer Attilla Danko, the site is supported by individuals or clubs who usually sponsor a map for their particular observing site. If your local site isn't up there, it's well worth becoming a sponsor to have it added.