In our opinion, the primary reason for purchasing one of these three cameras is the availability of additional lenses. Each of these cameras is available either as a body-alone or a kit that includes one lens. We cannot recommend purchasing any of these cameras as a body-only purchase, unless you already have lenses that fit these cameras.
The Canon T1i kit ships with the EF-S 18 mm-55 mm ƒ3.5-ƒ5.6 lens. There are seven lenses in the EF-S line available from Canon, ranging from the EF-S 10 mm-22 mm ƒ3.5-ƒ4.5 wide-angle zoom to the EF-S 55 mm-250 mm ƒ4-ƒ5.6 telephoto zoom. The EF-S line also includes a 60 mm ƒ2.8 macro lens. The important feature of these lenses is that they are designed and scaled to the size of the imager in this camera. In other words, there will be no multiplier effect as with the EF lenses from Canon. You can still use the Canon EF-series of lenses, but those will be affected by the multiplier affect (a problem that occurs when the imager of a camera doesn't match up to the lens).
Nikon has several more lenses available for the D5000, which were designed with the camera in mind. The D5000 kit ships with the AF-S DX Nikkor 18 mm-55 mm ƒ3.5-ƒ5.6G VR lens. The VR stands for vibration reduction, dampening the camera's vibration in slow speed or low light conditions. Nikon also has eight other DX zoom lenses as well as two other specialty lenses with the D5000 in mind.
Olympus ships the E-620 kit with its 14 mm-42 mm ƒ3.5-ƒ4.6 ED lens. Where Canon and Nikon have always included their lenses in their regular lens selection, Olympus lenses have always been designed for the smaller body/small imager of their SLR cameras. Olympus differentiates their lenses by grade.
The Super High Grade lens group includes four zoom lenses and two fixed focal length lenses. This group is noted for the best optics available and range in price up to $5,000, all aimed at the professional photographer. The High Grade groups have eight lenses and a tele-converter, with several zoom lenses and a few fixed foal length lenses. The Standard Grade group is the group that our kit lens ships from. Containing nine lenses, this group has six zoom lenses and three fixed focal length ones.
For lenses, all three cameras receive a 4.