Many companies are making tablet PCs, with more popping up all the time. At this writing the iPad from Apple is the most popular tablet, running its iOS operating system, but there are several tablets running the Google Android system as well. Samsung makes some through ATT that allow for 3G coverage. So that's a plus when Wi-fi is not an option. Dell and many other are getting into the market as well. Computer companies would be silly not to jump on the tablet PC bandwagon, which has barely began to take off.
Apple is the undisputed king of the tablet PC market right now. Although lots of companies are making their own version of tablet computers, Apple still leads the way in sales by a wide margin. The funny thing is that the best tablets on the market aren't even produced by Apple, but Apple really helped itself out by innovating the technology and getting out in front with it. When people think of tablet PCs, its really hard to not think of the iPad and what it's done for the industry. The same thing happened when Apple released the iPhone out in front of the rest of the smartphone market. Other companies produced superior products, but it's taken customers awhile to warm up to buying gadgets that aren't made by Apple.