Making technical reports available I'm in the field of engineering, electrical engineering specifically. I find myself with several technical reports and notes I've written over the years, and some colleagues have asked me for these. I think these may be beneficial to others, too and I'd like to make these publicly available somehow. 

I've refrained from using the word "publishing", because I don't ever intend to submit it to a conference or a journal - the type of material just isn't right. See here http://www.mers.byu.edu/docs/reports/MERS9401.pdf for an example (I'm not the author of this particular note). arXiv doesn't cover electrical engineering topics; also, I doubt my reports would qualify "preprints", because preprints, as I understand, is something the author intends to publish eventually.

I know some folks who post these on their websites, but the danger is that self-maintained websites may eventually disappear, and with the websites, the reports themselves. Is there a good solution we have about this today? Perhaps a public repository?
If you are affiliated with an academic institution, many have some sort of mechanism for publishing tech reports, e.g. via DSpace.  This is the ideal solution, since it is designed for exactly the sort of publication that you are looking for.

Failing that, I would suggest looking a little bit closer into arXiv.  Many of the topics are quite vague, and depending on the topic, your work might well fit into something like cs.IT (Information Theory) or cs.SY (Systems and Control), or a number of other areas.

Finally, a possible fallback is to simply publish via a fairly stable repository like GitHub, which means it is at least not dependent on your own server, but an institution that is intended to be maintained over many years.