Software downloads:
First, your webmaster is not very plugged into the Mac world. If you find incredibly good free astronomy software for the Mac, let me know. You can arrange to show it to me on your machine, and if suitable, I can post a link to it here. With the advent of OS-X/Cocoa/Darwin, I expect to see much more development, and ports of existing Linux/Unix packages to the Mac. Much of this is free and of high quality.
Second, there are MANY astronomy programs out there, especially for the Windows platform. I don't have time to review them all, and the links here are to those that I have checked out recently. I will post mostly freeware listings, but will include some links to pay software that is exceptional and not advertized. The listings will probably grow (slowly) over time.
Celestia is a free real-time space simulation that lets you experience our universe in three dimensions. Unlike most planetarium software, Celestia doesn't confine you to the surface of the Earth. You can travel throughout the solar system, to any of over 100,000 stars, or even beyond the galaxy. All travel in Celestia is seamless; the exponential zoom feature lets you explore space across a huge range of scales, from galaxy clusters down to spacecraft only a few meters across. A 'point-and-goto' interface makes it simple to navigate through the universe to the object you want to visit.
Stellarium is a free software available for Windows, Mac and Linux. It renders 3D photo-realistic skies in real time. With Stellarium, you really see what you can see with your eyes, binoculars or a small telescope.
Partiview (Particle View) is a software package that allows one to navigate through three dimensional data sets. These data sets are not limited to astronomical data, but can be particle data of any type. Partiview is also able to display 2-D images as well as play prerecorded flight paths. The user interface is command line style, with tabs to click to enable/disable display options. MilkyWay object data is provided. Partiview is available for Linux, Windows, and Mac OS X.
Cartes du Ciel comes in a number of languages, and is a highly configurable and very useful planetarium program that can also produce observing charts. External catalogs can be added and the display configured to suit the user. It also connects to several popular telescope controllers. Well worth a look.
This is a fairly user friendly and very complete planetarium/star mapping program. Here you will find the planetarium program for Windows 95 and later. It is complete with the SAO, PPM, Tycho-2 star databases, up to date deepsky database containing 26000 objects , Realsky (r) images and access to the GSC 1.1, 1.2, and USNO CDROM databases. Also a LX200 control program, a Realsky viewer (incl. source) DDE controlled from HNSKY and a BMP/ JPEG to FITS conversion utility.
C2A (Computer Aided Astronomy) is planetarium software that allows you to build detailed views of stellar fields. An important objective of this software is to take into account the main catalogues available to professional and amateur astronomers in order to prepare observations on small fields as well as astrometry works. C2A has numerous functions including planetary positions, asteroid and comet path display, ephemeris, and more.
Ephemeris for X-windows. A 'scientific grade' astronomy package with a more daunting interface, but powerful enough to be in service at major observatories around the world. XEphemeris will do amazing searches and has many features that will take some time and work to take advantage of. Available for free to compile under Linux/FreeBSD/Solaris/Unix, or for purchase for Mac OS-X and Unix/Linux at $49.95.
Astronomy software review and link sites:
Probably the best starting point for Mac owners/users. It selects and rates astronomy programs for Macs using a scale of 1 to 4 stars and written descriptions, and has information on the programs (including requirements for Hypercard, OS version, hardware, etc.)
Is a site maintained by Hugo D. Valentim that reviews and links to freeware and shareware astronomy programs, with 22 Planetarium programs alone. It's a good jumping off point for discovering your own favorites.
Maintained by Bill Arnett, keeps track of astronomy related software sorted by platform: Mac, Windows, Linux/Unix/X-windows, PalmOS, and OS/2 WARP. It also links to web-based planetariumm programs, other lists of astronomy software, and data sources.