These are the top 100 languages in order of number of first language speakers.

This is from the 13th Edition of the Ethnologue (1996).
More up-to-date information may well be avaliable

www.ethnologue.com is by far, the best language resource I've found on the web and much more detailed information about individual languages is avaliable there.

Top 100 Languages by Population

RankLanguage NamePrimary CountryPopulation
1CHINESE, MANDARINChina885,000,000
2SPANISHSpain332,000,000
3ENGLISHUnited Kingdom322,000,000
4BENGALIBangladesh189,000,000
5HINDIIndia182,000,000
6PORTUGUESEPortugal170,000,000
7RUSSIANRussia170,000,000
8JAPANESEJapan125,000,000
9GERMAN, STANDARDGermany98,000,000
10CHINESE, WUChina77,175,000
11JAVANESEIndonesia, Java, Bali75,500,800
12KOREANKorea, South75,000,000
13FRENCHFrance72,000,000
14VIETNAMESEViet Nam67,662,000
15TELUGUIndia66,350,000
16CHINESE, YUEChina66,000,000
17MARATHIIndia64,783,000
18TAMILIndia63,075,000
19TURKISHTurkey59,000,000
20URDUPakistan58,000,000
21CHINESE, MIN NANChina49,000,000
22CHINESE, JINYUChina45,000,000
23GUJARATIIndia44,000,000
24POLISHPoland44,000,000
25ARABIC, EGYPTIAN SPOKENEgypt42,500,000
26UKRAINIANUkraine41,000,000
27ITALIANItaly?x'37,000,000'
28CHINESE, XIANGChina36,015,000
29MALAYALAMIndia34,022,000
30CHINESE, HAKKAChina34,000,000
31KANNADAIndia33,663,000
32ORIYAIndia31,000,000
33PANJABI, WESTERNPakistan30,000,000
34SUNDAIndonesia27,000,000
35PANJABI, EASTERNIndia26,013,000
36ROMANIANRomania26,000,000
37BHOJPURIIndia25,000,000
38AZERBAIJANI, SOUTHIran24,364,000
39FARSI, WESTERNIran24,280,000
40MAITHILIIndia24,260,000
41HAUSANigeria24,200,000
42ARABIC, ALGERIAN SPOKENAlgeria22,400,000
43BURMESEMyanmar22,000,000
44SERBO-CROATIANYugoslavia21,000,000
45CHINESE, GANChina20,580,000
46AWADHIIndia20,540,000
47THAIThailand20,047,000
48DUTCHNetherlands20,000,000
49YORUBANigeria20,000,000
50SINDHIPakistan19,720,000
51ARABIC, MOROCCAN SPOKENMorocco19,542,000
52ARABIC, SAIDI SPOKENEgypt18,900,000
53UZBEK, NORTHERNUzbekistan18,466,000
54MALAYMalaysia, Peninsular17,600,000
55AMHARICEthiopia17,413,000
56INDONESIANIndonesia17,050,000
57IGBONigeria17,000,000
58TAGALOGPhilippines17,000,000
59NEPALINepal16,056,000
60ARABIC, SUDANESE SPOKENSudan16,000,000
61SARAIKIPakistan15,015,000
62CEBUANOPhilippines15,000,000
63ARABIC, NORTH LEVANTINE SPOKENSyria15,000,000
64THAI, NORTHEASTERNThailand15,000,000
65ASSAMESEIndia14,634,000
66HUNGARIANHungary14,500,000
67CHITTAGONIANBangladesh14,000,000
68ARABIC, MESOPOTAMIAN SPOKENIraq13,900,000
69MADURAIndonesia, Java, Bali13,694,000
70SINHALASri Lanka13,220,000
71HARYANVIIndia13,000,000
72MARWARIIndia12,104,000
73CZECHCzech Republic12,000,000
74GREEKGreece12,000,000
75MAGAHIIndia12,000,000
76CHHATTISGARHIIndia10,985,000
77DECCANIndia10,709,800
78CHINESE, MIN BEIChina10,537,000
79BELARUSANBelarus10,200,000
80ZHUANG, NORTHERNChina10,000,000
81ARABIC, NAJDI SPOKENSaudi Arabia9,800,000
82PASHTO, NORTHERNPakistan9,685,000
83SOMALISomalia9,472,000
84MALAGASYMadagascar9,398,700
85ARABIC, TUNISIAN SPOKENTunisia9,308,000
86RWANDARwanda9,306,800
87ZULUSouth Africa9,142,000
88BULGARIANBulgaria9,000,000
89SWEDISHSweden9,000,000
90LOMBARDItaly8,974,000
91OROMO, WEST-CENTRALEthiopia8,920,000
92PASHTO, SOUTHERNAfghanistan8,206,000
93KAZAKHKazakhstan8,000,000
94ILOCANOPhilippines8,000,000
95TATARRussia8,000,000
96FULFULDE, NIGERIANNigeria7,611,000
97ARABIC, SANAANI SPOKENYemen7,600,000
98UYGHURChina7,595,512
99HAITIAN CREOLE FRENCHHaiti7,372,000
100AZERBAIJANI, NORTHAzerbaijan7,059,000
101NAPOLETANO-CALABRESEItaly7,047,400
102KHMER, CENTRALCambodia7,039,200
103FARSI, EASTERNAfghanistan7,000,000
104AKANGhana7,000,000
105HILIGAYNONPhilippines7,000,000
106KURMANJITurkey7,000,000
107SHONAZimbabwe7,000,000