I personally use Microsoft Outlook Express, and have found it to be totally sufficient for my needs. Just pick one that suits your needs. The closing of the letter - this is the first four lines you see in the signature above. It should in someway catch the reader's eye or stir up some emotion that will impel them to act. The information which shows the reader how to act - you have to include some form of information about how an interested reader can access a deal, contact someone, visit a web site, or whatever else you're trying to get that reader to do.
Signatures can be a powerful tool for the budget-conscious web site owner, if used effectively.
If used poorly, they're simply a waste of time and effort. Don't just send one signature you've developed over and over again to the same people. Think of several different ones you could use for your business, and alternate. Also, signatures are an excellent tool for promoting an affiliate program that you're involved in. Again, have as many as you can, and rotate.