SNPEffect database is very useful for me to assess various aspects of non-synonymous SNPs. SNPEffect is an integrated resource that run individual sequence and structure based prediction algorithms to predicts whether SNP affect various features.
Sequence features: Subcellular localisation, Turnover rate, Myristoylation, PTS1, Type I Geranylation, Type II Geranylation Farnesylation, GPI Anchor, Acetylation, O-glycosylation, N-glycosylation, Phosphorylation, Propeptide cleavage sites, Signal peptides, Nuclear export signal
Structural features: Aggregation, Stability, Amylogenic regions, Transmembrane regions, Active Sites, Hsp70 Binding
IMHO, this resource can give you more insight in to the effect of SNP in protein beyond the limited information (for example: deleterious effect) provided by tools like SIFT, PolyPhen or PMut.
References: SNPEffect V1, SNPEffect V2