As you may know our marketplace has search functionality. Here is the statistics of popular search terms (top 100) for the last several months. Perhaps it could be helpful for third-party plugin vendors. The second column is number of requests.
discount 1637
paypal 1383
facebook 1172
payment 1139
google 1108
chat 1001
filter 808
Slider 736
spanish 717
seo 549
menu 504
bootstrap 497
amazon 486
shipping 475
Nop-Templates.com 464
ebay 462
sms 458
Russian 456
product 454
search 431
stripe 428
pay 382
language 369
payu 335
Widget 331
theme 325
chinese 319
pagseguro 318
checkout 307
german 305
responsive 304
arabic 299
persian 290
sagepay 284
sage 274
nopadmin 273
content 272
ideal 271
tax 269
vendor 266
turkish 262
cms 255
alipay 254
fashion 240
image 239
zoom 236
eway 234
import 226
mailchimp 219
correios 214
rus 214
quickbooks 213
ribbon 212
pdf 212
nivo 205
attachment 200
dutch 199
italian 192
swedish 192
sociAL 190
UPS 189
invoice 182
ccavenue 182
feed 176
email 172
affiliate 171
video 170
french 166
banner 164
api 160
admin 156
klarna 155
worldpay 155
cart 153
india 151
dhl 150
price 150
mobile 147
blog 147
analytics 143
portuguese 140
sales 138
gallery 136
food 135
order 135
html 131
twitter 130
danish 130
request 130
noproot 129
furniture 129
ogone 128
mail 127
crm 125
farsi 124
erp 123
china 123
Authorize 121
nop-templates 117
hebrew 117
UPDATE (January 2017). Please find the updated list here