ETFs and retirement

Hi, all. I've been looking into ETFs as a result of responses I got to
an earlier question regarding retirement investing. They look kind of
cool, but are they something suitable for retirement investing and
such? In other words, can you buy them in an IRA capacity?

If not, would it generally be inadvisable to invest in ETFs when I'm
not maxing out either my 401k or my Roth IRA? My 401k is funded up
through what my employer matches, but no more, and I'm also
contributing to but not maxing out a Roth IRA. However, I can probably
scrape together another hundred bucks/month or so for investment
purposes, and although I'd like to put that money toward ETFs, I'm
wondering if it'd be wiser to put it toward the IRA.

Thanks for any help on the matter.