Newer-model bike similar to Raleigh M50
A Raleigh M-50 was a great entry-level mountain bike. In order to answer your question well, let me clarify for you what buying a bike "like an M-50" means to me:


Inexpensive, but good quality:
  A "real" bike, built by a professional, which happens to be cheap.
Off-road capable:
  Regardless of whether you need or use this option, it feels good.
Commute friendly:
  To be honest, the M-50 wasn't a bike I'd have picked to commute on, and there are better options now as well. But you will compromise the off road capability if you pick something else.
Fun.


There are a few bikes to look at. Pretty much every quality brand has a similar model of MTB, with some variation in spec and price.

The first thing to decide is if you really want an off road bike, or just a durable one.

If you want an off road bike, meaning that you want to ride in dirt, gravel, or non-pavement situations on at least a part time basis, look at the Scott Aspect series. The price range is good, an in just about every respect, I would consider them a modernized M-50 type bicycle.

Also, the Specialized Hardrock series.
And the Trek Sport series.

If you really want a commuter, which is as tough and as durable, and as simple to use and maintain as a mountain bike, but with the speed, agility, and distance friendliness of a road bike, then look at:

The Scott SUB series (Particularly the SUB 10)

or 

the Specialized Sirrus (Less durable, but a great commuter).

You need to find the bikes in a LBS and ride them, which will tell you more than I ever could here, but I hope that helps, and feel free to ask more questions if I can do anything more.