Want to find an easier way to receive packages through the mail? Look to BufferBox, a Y Combinator startup developing a drop-off service for packages. Its heavy metal locker is installed at three Google campuses already.

Hope to do better in college? College tutoring service Study Edge might be just what the professor ordered. Ten thousand college students already pay for its help, generating revenue for the young company of $1.7 million during the past year.

Then there is HD Trade Services, a maker of warehouse inventory software, and Grid, working on a spreadsheet for tablets. More than you might expect, Y Combinator’s current crop of demo day companies addressed real problems experienced by real people.

Sure, there were companies targeting today’s big Silicon Valley trends, mobile video, for instance, and news and content distribution with a social Web orientation. And there were celebrated investors roaming the floor. Ron Conway showed up and so did Ashton Kutcher.

But a good number of companies presenting business plans to an audience of 500 or so founders and investors at Tuesday’s Y Combinator demo day in Mountain View, Calif., had a down to earth focus and a sense that the back office can ultimately be more lucrative than the front.

“I was really impressed by some of them,” said angel investor Alejandro Rivas Micoud, taking a break from the throng of activity. Some “walked out on stage and blew my socks off.”

The atmosphere proved a good one for companies seeking new backers. BufferBox co-founder (??) Mike McCauley said he received interest in an A Round, even though he closed a seed round last week. “We just have a lot of interest from a lot of big guys,” he sad, referring to top tier firms he declined to name.

It seemed likely some of these firms would bite. “I always find something” to invest in, said angel investor Raj Sandhu, who was attending his fourth demo day. Y Combinator co-founder “Paul (Graham’s) a class act.”

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