Cricket Wireless is the king of cheap-o mobile services. They now offer a wireless broadband service for only $40/month (or $35/month if you already have a Cricket phone), which is $20 less than the major players in the industry.
Just in case you aren't familiar with Cricket, it's a cellular service provider that doesn't require you to sign into a contract, as any other (non-prepaid) wireless provider does. Instead, you pay full price for your equipment up-front and then pay for the service on a month-to-month basis as you like.
Cricket currently offers only one equipment choice, the UTStarcom um100c, pictured below. It costs about $135, with activation, but right now there is a mail-in rebate of $50. The first month of service is included in that price. From then on it is $40/month. It's wider than my Sprint wireless broadband card, but it's still a very portable device. An extension cable is included but it's pretty bulky.
In Portland I can download at about 70KB/sec, or about the speed of an old home (A)DSL connection, and less than a half the speed of my Sprint card. The upload speed is pretty low.
That said, it is completely useable for general Internet surfing and email checking. The bandwidth is high enough that most online radio stations and video sites will work without issue, though load times might be slightly higher than they would through a wired broadband connection. Add the fact that you now have an Internet connection that you can take with you on the go, and that $40 begins to look like a pretty good bargain.
Cricket's network is definitely not national. You can see their coverage maps here. Major markets are completely dark; for example, I can't use the card in Los Angeles, New York or Chicago at all, but I can in Modesto?? It's a long story regarding their expansion strategy, which cities they choose to set up shop in and which they don't. The short story is that, if you travel at all, you will probably want a card from Sprint or some other major wireless player.
The software included for the Mac is terrible. Once I installed it, my Sprint card refused to work. See my post on circumventing the software. I haven't tried the card at all on the PC, nor do I have any desire to do so.