As other responders said, you can certainly roll over multiple 401(k) into a single account.
An added benefit of such rollover (besides the ease of tracking) is that you can shop around for your Rollover IRA provider and find the one that gives you the specific mutual funds that you want to invest in, the lowest expenses, etc. - in short, find the best fit to your priorities. There are also "lemon" 401(k) plans and if you are in one of them, get out! And rollover is the way out.
There is also one possibility to keep an eye on as it happens rarely, but it does happen - rolling a 401(k) over into another 401(k). I've done it once when I started a job at a company that had a great 401(k) with a good selection of low-cost mutual funds. I rolled the 401(k) from one previous job in to this 401(k) to take advantage of it. At the same time I kept a Rollover IRA, combining the 401(k) from all other jobs; it had more investment options and provided some flexibility.