Standard datasets for testing new multiple sequence alignment algorithms?
I would suggest you PAcAlCI or Prediction of Accuracy in Alignments based on Computational Intelligence, though the acronym in wierd the tool is good for testing new Sequence Alignments. They 

But before you start testing your algorithm, I suggest take a look at these papers:

[1] Who Watches the Watchmen? An Appraisal of Benchmarks for Multiple Sequence 
Alignment

[2] Issues in bioinformatics benchmarking: the case study of multiple sequence alignment

PS: I haven't tested or used the tool myself.