If you want to play Metal Gear Solid: Peace Walker, and don't mind a little Japanese, the game's official website has a demo available to all.
Metal Gear Solid: Peace Walker is the series' newest upcoming PSP iteration. The game's lengthy Tokyo Game Show trailer gave us a good indication that it would be one to look out for, as if the Metal Gear name alone wasn't enough. If you'd like to try Peace Walker's latest demo, it is available for download on the game's official Japanese website if you don't have a Japanese PSN account. If you've already played the demo made available around the time of TGS 2009, this is a new version with a few small changes such as Snake's roll controls and the camouflage color of certain enemies.
To make things a little easier, the download link is the big orange box on this page. Your PSP must be updated with firmware version 6.00 to play. After downloading the file, unzip it, connect your PSP to your computer with a USB cable, and put the file from the zip's "GAME" folder into your PSP's "GAME" folder. The demo is a little over 100 MB large, so you must have an appropriate storage device.
The demo's dialogue is all in Japanese, but it's not too hard to pick up Peace Walker's controls. The game has a tutorial (which can be skipped) that gives on-screen prompts as to how to perform certain actions, like Snake's satisfying hip throw. Peace Walker's controls are also available here in English if you click on "Game System." The game has multiple missions, plenty of weapons, and is full of guards to knock unconscious. Just don't forget that in Metal Gear the circle button is used to confirm choices rather than X.
I haven't yet gotten to play Metal Gear Solid: Portable Ops, so Snake feels somewhat strange at first to control on the PSP. The analog nub is not comfortable when slightly pushing it to make Snake sneak quietly. Still, it's nice to get a taste of Hideo Kojima's future PSP Metal Gear, even though most of us will probably have to wait for an English translation to understand any of the storyline.