As the web developed CS departments started putting their tech reports online. But why should you have to go to individual department web sites to track down each report? So people developed aggregators like NCSTRL that collected pointers to the individual paper and let you search among all of them. CiteSeer went a step further, automatically searching and parsing technical reports and matching citations.
But why have technical reports divided by departments? We each live in two communities—our university and our research field. It's the latter that cares about the content of our papers. So now we see tech report systems by research area, either area specific systems like ECCC or very broad report systems like arXiv that maintain specific lists in individual subareas that bypass the department completely.
What's next? Maybe I won't submit a tech report at all letting search engines like Google Scholar or tagging systems like CiteULike organize the papers. Departmental tech reports still exist but don't play the role they once did and who can predict how we will publish our new results even five or ten years down the road.