Expression systems
Mainstream systems
Escherichia coli
Rosetta cells
If you are trying to express non-bacterial protein, it is quite likely that your gene of interest contains rare codons and thus E.Coli can't express it well. Rosetta and Rosetta 2 cells (available from Novagen) are designed to overcome this problem. For human proteins, expression levels may sometimes change from "I need to enrich the protein batch on affinity column before I can see protein on the gel" to "80% of the protein in the lysed cells is my protein of interest".
Fungi
Pichia pastoris
Insect cells
Baculovirus-mediated expression in Spodoptera frugiperd and Trichoplusia ni cells
Mammalian cells
Chinese hamster ovary (CHO) cultured cells
Human Embryonic Kidney (HEK) 293 cultured cells
Other systems
Bacteria
Bacillus subtilis
Bacillus megaterium
Pseudomonas fluorescens
Protozoa
Leishmania tarentolae
Fungi
Saccharomyces cerevisiae
Schizosaccharomyces pombe
Hansenula polymorpha
Filamentous fungi
Plant cells
Algae
Chloroplasts
Nicotiana tabacum