This database accompanies 'PHOSIDA (phosphorylation site database): management, structural and evolutionary investigation, and prediction of phosphosites',
Florian Gnad, Shubin Ren, Juergen Cox, Jesper V Olsen, Boris Macek, Mario Oroshi, Matthias Mann (2007); Genome Biology.
An update of the database is described in 'PHOSIDA 2011: the posttranslational modification database', Florian Gnad, Jeremy Gunawardena, Matthias Mann (2011); Nucleic Acids Research.

Phosida allows the retrieval of phosphorylation, acetylation, and N-glycosylation data of any protein of interest. It lists posttranslational modification sites associated with particular projects and proteomes or, alternatively, displays posttranslational modifications found for any protein or protein group of interest. In addition, structural and evolutionary information on each modified protein and posttranslational modification site is integrated.

Importantly, Phosida links extensive peptide information to the sites, such as several peptides implicating the same site and temporal profiles of each site in response to stimulus (e.g., EGF stimulation).

A test version of a new PHOSIDA release, which summarizes quantitative data at the site level (for human datasets), can be found here.
Proteins Peptides Sites
PHOSPHORYLATION
Homo sapiens 8283 23130 24262
Mus Musculus 9234 24604 25085
D. melanogaster 2379 8777 10043
C. elegans 2373 6926 6780
S. cerevisiae 1118 4457 3620
Lactococcus lactis 63 99 73
Bacillus subtilis 78 102 76
Escherichia coli 79 104 81
H. salinarium 62 100 75
ACETYLATION
Homo sapiens 1750 4219 3600
N-GLYCOSYLATION
Mus musculus 2352 8681 6367