Free card making software is NOT easy to find online.
The major software packages don't do free trials as far as I'm aware and the others that are 'free' or 'shareware' can be a little basic.
Picasa is great for basic editing but unfortunately, you can't actually 'put together' a card using it.
However, you CAN put together a card using the free downloadable trial from Scrapbook Max! Ignore the word 'scrapbook' and just think 'cards'...same principle, just smaller!
My first software was Greeting Card Factory Delux, did the job, but I felt restricted, so I moved on to Paint Shop Pro which gives you much greater flexibility and design features, but it's a bit more 'advanced' graphic design software.
Enter Scrapbook Max! Not only can I put together actual cards on screen to print straight off or send digitally, I can use it do design patterns and printables for making cards up the traditional way, with glue!
And even better - there's a free trial! Whoot! If you buy it afterwards (and it's only $39, about £20) you get heaps more goodies shipped to you on a CD.