MDB is the format used by Microsoft Access, one of the most popular databases in the world. Usually, a front end application of some type is used to--no pun intended--access the data. However, it can sometimes occur that you end up with a "naked" MDB and no way to peek inside it. Enter MDB Viewer Plus (free).
MDB Viewer Plus is a lightweight (no fancy install, just extract the executable) program which can be easily stowed on a USB key. It has a sparse interface, with nothing more than brief guides to filtering and sorting as help or documentation. For most tasks, you must poke around to figure out what to do. Fortunately, the program follows most standard Windows conventions, so it is not difficult to use.
Unfortunately, it has a number of quirks which make it less than useful. Opening an MDB which contains many tables produces a very long tab bar of table names, arranged alphabetically. You cannot rearrange the tabs, or hide specific tables (you can hide columns within a table, however). Thus, if the tables you are most interested in are on opposite sides of the table, you will have a lot of scrolling back and forth to do.
Other features seem to work sporadically. The "Record View" feature, useful for editing data in a table with many columns, works on smaller tables but produces only a blank window on larger ones. At one point in my informal tests, MDB Viewer Plus started throwing up error messages about missing columns, and the only way to restore functionality was to quit and restart. One useful-seeming feature, the ability to create a blank database and begin adding tables to it (making MDB Viewer Plus a quick-and-dirty way to make and populate an Access database) was plagued with bugs.
It's not all bad. MDB Viewer Plus allows for filtering and sorting, which are fairly intuitive, and also for editing, not just viewing, of data, making it a useful tool for simple database maintenance, especially if it's not possible to get to the data file through a more normal front end. Given some of the bugs, though, this is a tool of last resort. Use it when there's nothing else which will get you to your data.