Evapo-Rust Web Site
It's a rust removal product. Immerse rusty parts in Evapo-Rust for a few hours or overnight and the rust is dissolved into the solution. The stuff works really well. It's available at a number of retailers, including Advance Auto Parts, O'Reilly, and Harbor Freight Tool. The down side is that it's not super cheap -- about $25 per gallon if I remember correctly. It's reusable, though, so you can make that gallon go a long way.
Oxalic acid is another thing you can use similarly. Immerse the rusty item and the rust just dissolves. Oxalic acid is often sold as "wood bleach" at hardware stores. It's a lot cheaper than Evapo-Rust, so it makes sense to use it for big items. An eight-buck tub of oxalic acid crystals mixed with a kiddie pool full of water makes a great rust removal bath for bike frames. It's just as effective, too, but maybe not quite as convenient since you have to mix it (just add water). Also, oxalic acid can remove black oxide finishes that you might find on some bike parts like headset cups and freewheel cogs.
Both of these have been discussed quite a bit, along with other rust removal techniques on Bikeforums, especially in the Classic & Vintage subforum. You could probably Google it and get a ton of results -- just add
site:bikeforums.net to your Google query to get results from this site.