For nouns ending in s (whether singular or plural), the possessive form requires only an apostrophe:

Unisys' annual report

Phyliss' comments

your boss' agenda

the actress' role
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NOTE: if a singular noun ending in s is followed by a word beginning with s, use only the apostrophe, not 's. Charles' shirt


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Note added at 11 mins (2004-03-05 17:13:26 GMT)
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There is obviously more than one point of view on the topic :-)

Many writers consider it bad form to use apostrophe -s possessives with pieces of furniture and buildings or inanimate objects in general. Instead of \"the desk\'s edge\" (according to many authorities), we should write \"the edge of the desk\" and instead of \"the hotel\'s windows\" we should write \"the windows of the hotel.\" In fact, we would probably avoid the possessive altogether and use the noun as an attributive: \"the hotel windows.\" This rule (if, in fact, it is one) is no longer universally endorsed. We would not say \"the radio of that car\" instead of \"that car\'s radio\" (or the \"car radio\") and we would not write \"the desire of my heart\" instead of \"my heart\'s desire.\" Writing \"the edge of the ski\" would probably be an improvement over \"the ski\'s edge,\" however.

For expressions of time and measurement, the possessive is shown with an apostrophe -s: \"one dollar\'s worth,\" \"two dollars\' worth,\" \"a hard day\'s night,\" \"two years\' experience,\" \"an evening\'s entertainment.\"