A mechanic friend of mine told me that I have a crack in my engine block. I have all the signs. White smoke coming from the exhast, I can put antifreeze in my car and the next day its gone, there is antifreeze mixing with my oil, the car will suddently stop on me or not start at all, and there is too much oil in my car. He told me that block sealer would probably do a good job of keeping it together long enough for me to either pay for the repairs or get a new engine. My question is how long do block sealers work and what is the best one to use. All of your insight would be really great for me.

Some sealers work well enough, temporarily.
Some don't work well at all.
I don't endorse these magic cures, so I don't know which to recommend.
The best option for your particular failure may be quite different for the next type of failure or engine material: what works on one type block/failure may not work at all on another block with a different kind of failure.
Research will be important here. Read labels on a few sealers and do some research online to find out what products are available, what they claim to fix and how do they do it.
Be sure to read their disclaimers, too. You may find they have less confidence in their own product than you hoped for.

Cracked block or is it failed head gasket or intake gasket. This would be a easy and less money to repair then a cracked block. IS he sure it's a block? That dosen't happen to often, my guess would be gaskets first... maybe a cracked head also. Also that antifreze will eat the bearings away so if you have any thought of keeping this don't run it